This concerns a data communication system using frequency division multiplexing (FDM) and, in particular, a method and apparatus for control signaling in such a system.
A specific use for our invention is in private line data networks in which several remote terminals are connected over narrow band private lines to a central processing unit (CPU). Such a system typically is used in conjunction with a public telephone network, such as that of the Bell System, to provide groups of telephone subscribers in each of several localities with low-cost, long-distance data links to a centrally located computer. Thus, the whole system comprises a multitude of subscriber telephone and data sets, a public telephone network, several remote terminals, a private line network, and a central processing unit.
As is well known, data is transmitted over conventional public telephone networks in the form of pulses of certain frequencies. At the transmitter, a modulating device called a data set, or modem, is used to convert a DC signal representative of a stream of digital data, which may be received from any type of digital data processing machine, into an AC signal representative of this same stream of digital data. At the receiver, another modem converts received AC signals back to digital DC signals. Ordinarily, data communication takes place in both directions on a telephone line and each modem is equipped both to convert DC signals to AC signals that are transmitted and to convert received AC signals to DC signals. Thus, in a typical data communication system, each subscriber has at least one modem transmitter/receiver and each remote terminal of a private line data netwwork has at least one modem transmitter/receiver.
In modems that are presently used with data communication systems, one of the two DC levels that represents digital data is converted by a modem to an AC signal having a first frequency; while the other level of the DC signal is converted to an AC signal having a second frequency. It is conventional in the art to refer to one of these DC levels and the corresponding AC frequency as a SPACE or 0, and to the other DC level and the corresponding AC frequency as a MARK or 1. To minimize interference between signals that are transmitted from a terminal and the signals that are received at that terminal and to permit communication between more than two terminals, it is customary for a modem to transmit MARK and SPACE signals at frequencies that are centered about a first center frequency and to receive MARK and SPACE signals at frequencies that are centered about a second center frequency.
Extensive description of the operation of modems may be found in James Martin's book Telecommunications and the Computer, (Prentice Hall, 1969); in patent application Ser. No. 194,270, now U.S. Pat. No. 3,769,454, filed Nov. 1, 1971 by R. A. Liberman, W. C. Bond, and E. J. Soltysiak, entitled "Method and Apparatus for Testing Teletypewriter Terminals," and assigned to General DataComm Industries, Inc.; and in the Bell System Data Communications Technical Reference entitled "Characteristics of Teletypewriter Exchange Service," (September, 1970) available from: Engineering Director -- Data Communications, American Telephone and Telegraph Company, 195 Broadway, New York, N.Y. 10007.
Data is transmitted over the private line portion of the data communication network by methods such as frequency division multiplexing (FDM) that allow several phone calls to be conducted simultaneously over a single private line. In an FDM system, this is accomplished by transmitting each call within a specified frequency channel on the private line. At the remote terminal, an FDM transmitter/receiver converts DC signals from the remote terminal modem to signals having frequencies within the specified frequency channel; and it converts signals received from the CPU to DC signals that are applied to the remote terminal modem. A second FDM transmitter/receiver, which may be termed a local FDM, is located adjacent the CPU. This local FDM transmitter/receiver converts signals received from the remote FDM to DC signals that are applied to the CPU; and it also converts signals from the CPU to signals having frequencies within the frequency channel assigned for transmisssion to the remote FDM. The local FDM also performs interfacing required between the data communication system and the CPU. Extensive discussion of frequency division multiplexing may be found in the above-referenced Telecommunications and the Computer. As will be evident to those skilled in the art, the modulating and demodulating functions of an FDM transmitter/receiver are analogous to those of a modem.
In addition to converting signals from DC to AC and vice versa, modems provide control means for the communication system. Typical control signals of interest are: a DATA TERMINAL READY signal that indicates to a remote terminal modem that the CPU is prepared to receive data transmission from that modem; a RING signal that is a request from a subscriber's modem for a connection to the CPU; a DATA SET READY signal that indicates to the CPU that the remote terminal modem has answered a telephone call in response to a RING signal and is prepared to receive information from the subscriber's modem; an OUT OF SERVICE signal that indicates that the CPU is not operating; and a CARRIER DETECT that indicates the reception of the carrier signal at some point in the system. Because these signals are needed to connect the private line to the dial-operated public telephone network, these signals are referred to in the art as dial access controls. Also of interest in the discussion below is an ENERGY DETECT signal that indicates the reception of signal energy at some point in the system.
In prior art data communication systems, dial access control signals are transmitted over a private line between the CPU and the remote terminal modem as amplitude-modulates signals. This, however, creates problems in a narrow band channel such as that used for private line data communications. Specifically, the bandwidth of the amplitude-modulated control signals is sufficiently broad that distortion is created at the edges of the channel. This, in turn, makes it difficult to detect the control signals reliably.